Conventional electrode materials used in energy storage devices such as secondary battery systems include metal and metal oxide materials that have inherent limitations preventing their energy storage capacities from approaching their theoretical limits. The theoretical energy storage capacities of electrodes are further decreased by the need to add necessary amounts of binders, conductive particles and current collector structures. Electrodes are also limited due to the electrode material's inability to fully charge and discharge. Most conventional energy storage materials store electrical charges via chemical oxidation/reduction reactions on the surfaces of small particles. Often, the material located within the cores of these particles do not contribute to the energy density, thus the useable energy density of the material is much lower than that which is theoretically possible. As examples, the theoretical charge density of a pressed powder cadmium electrode is 477 mA-hr/g, but the practically achieveable discharge density is on the order of about 118 mA-hr/g; the theoretical charge density of an iron electrode is 960 mA-hr/g, but the actual discharge density of a commercial iron electrode is about 150 mA -hr/g; and the theoretical charge density of a zinc electrode is about 800 mA-hr/g, although the discharge density of such an electrode is generally about 160 mA-hr/g.
Conventional sealed energy storage systems are, by design, further self-limiting to avoid hazards such as hydrogen evolution. By way of example, a sealed nickel-cadmium cell incorporates an ampere-hour capacity of cadmium hydroxide that is about seventy to eighty percent greater than the ampere-hour capacity of the nickel hydroxide positive electrode. Upon charging this particular sealed cell, nickel hydroxide eventually becomes fully converted to a higher oxide of nickel if the charging current is continued after completed conversion of the nickel hydroxide, oxygen evolution commences and the system is said to be in an overcharge mode. Simultaneously during the charging process, as nickel hydroxide is being converted to a higher oxide of nickel at the positive electrode, cadmium hydroxide is being reduced to cadmium at the negative electrode. If all the cadmium hydroxide were reduced to cadmium metal, hydrogen evolution would commence immediately after reduction to the metal was completed and would continue to be evolved as long as the charging current persisted. The evolved hydrogen could eventually rupture the sealed cell. However, completed reduction of the cadmium hydroxide to cadmium metal is prevented because oxygen evolved at the positive electrode in the overcharge mode reacts with the excess cadmium metal in the negative electrode. Thus, an equilibrium is established whereby oxygen produced at the positive electrode on overcharge is consumed at the negative electrode, thereby preventing hydrogen evolution at this electrode. In this particular conventional sealed cell system, as well as many similar systems operating on the so-called oxygen cycle, hydrogen must not be allowed to be evolved, since there is no simple mechanism to oxidize hydrogen once formed in the cell. Thus, an important limitation of conventional nickel-cadmium cells is the need to have a large ampere-hour excess of cadmium hydroxide with respect to nickel hydroxide as a safety factor to insure that hydrogen is not formed in the cell. The energy density of the total cell is therefore diminished considerably since this large excess of cadmium hydroxide cannot be fully charged to realize its full capacity. This situation is described as a positive limited cell in the charge mode. For these reasons, energy storage devices using conventional electrochemical storage electrodes have charge storage densities that are substantially lower than their theoretical values.
Hydrogen-evolution during the discharge of alkaline electrolytic storage cells, especially cells connected in series was addressed by G. Neumann in U.S. Pat. No. 2,934,580 entitled "Electrolytic Cell, Particularly Accumulator Cell". Neumann recognized that an undesirable polarity reversal may occur in cells that become fully discharged, at which time hydrogen is irreversibly evolved. In accordance with the teaching of this reference, there is disposed between electrodes of opposing polarities an intermediate layer impregnated with electrolyte and containing at least one metallic compound having a relatively low conductivity. Such metal compounds are the oxides and hydroxides of the metals that form the active mass of an electrode; in the case of cadmium electrodes, cadmium oxide and cadmium hydroxide. When the polarity of the electrode in the cell becomes reversed, the metallic compound is reduced to the elemental metal instead of producing H.sub.2 and becomes electrically conductive. Newman also prefers each cell to contain a negative electrode having a greater amount of active material than that of the positive electrode so that the negative electrode remains still charged when the positive electrode is fully discharged. It has been found that such arrangements are not effective at high discharge rates and often cause internal shorting of the cell.
Others have approached this problem by dealing with hydrogen once it is formed, rather than attempting to prevent its evolution. U.S. Pat. No. 3,117,033 to F. Bachmann entitled "Sealed Alkaline Storage Battery with Hydrogen Absorbing Electrode" describes a hydrogen-absorbing electrode arrangement utilizing an auxiliary electrode which contains silver or a silver compound in electrical connection with a positive electrode. Bachmann recognizes that a first concern with this auxiliary electrode is that silver oxide may be soluble in the electrolyte and migrate to the negative electrode causing short circuiting of the cell. This concern is best solved by Bachmann by disposing the auxiliary electrode in electrical contact with the positive electrode and on the opposite side of the positive electrode that faces the negative electrode.
This solution is useful for limited irreversible adsorption of hydrogen to partially decrease the gas pressure accumulated within a sealed cell.
Free oxygen is also generated to some extent in most energy storage systems utilizing conventional electrochemical storage materials. The design of energy storage devices must account for oxygen generated in the device and present as gaseous oxygen or as oxygen dissolved in the electrolyte.
In rechargeable alkaline batteries, when the charge step approaches completion, evolution of oxygen normally occurs as a result of the parallel half-cell reaction, EQU 20H.sup.- --&gt;H.sub.2 O+1/20.sub.2 +2e.sup.-
As the positive electrode becomes fully charged, the above reaction becomes responsible for a considerable production of oxygen gas. Simultaneous evolution of hydrogen gas at the negative electrode may be avoided by an overdesign in the capacity of the negative electrode. This leaves the production of oxygen in the latter stages of charge and overcharge as a major problem since such production consumes hydroxyl ions thereby upsetting the electrolyte composition, including pH, and the electrochemical behavior of the battery cell or cells.
Some recombination of oxygen to form hydroxyl ion and a reduction in the amount of free oxygen present, may occur at the negative electrode during the aforementioned charge and overcharge portion of the cell cycle. The overall reaction for such recombination, for example with a bivalent negative electrode, may be written as follows: EQU O.sub.2 +2H.sub.2 O+2A-&gt;2A(OH).sub.2
where A may be a material such as cadmium, zinc or the like. The rate at which this recombination occurs, however, generally is insufficient to overcome the problem of oxygen generation particularly where a high rate of charging is involved in the use of the batteries.
Another problem which accompanies that of oxygen evolution is loss of some electrolyte through entrainment with the evolving gas. This may result in the additional deleterious effect of drying up the cell. Sealing of such an unbalanced cell is obviously dangerous since it could lead to early failure by bursting. Thus, in applications which call for a sealed cell, such a system may not be employed.
Recently, new classes of materials have been identified as having the ability to reversibly store energy through a hydrogen storage mechanism. Some of these materials are amorphous metal alloys. A general discussion of hydrogen adsorption by amorphous, or glassy, metal alloys was provided by G. G. Libowitz and A. J. Maeland, "Interactions of Hydrogen with Metallic Glass Alloys", Journal of the Less-Common Metals, 101, pp. 131-143, 1984.
Of the hydrogen storage amorphous metal alloys, copending patent applications USSN 717,429 and 717,428 to Tenhover et al. and Harris et al., now abandoned respectively, describe compositions and structures having outstanding hydrogen storage properties including the ability to be repeatedly fully charged and discharged. The measured charge densities of amorphous metal electrodes described in these patent applications range from about 200 mA-hr/g to about 444 mA-hr/g. These materials are active absorbers of hydrogen, and so hydrogen gas evolution in an energy storage system utilizing such materials is not a major concern. These materials have the ability to recombine oxygen, and so remove oxygen evolution as a major operating concern in a sealed system. Also, amorphous, or glassy, metal alloys do not exhibit phase changes such as dendritic growth over time, and so are more stable than some conventional electrode materials.
Others have suggested anodes for sealed secondary batteries that consist solely of a hydrogen-adsorbing material as the energy storing portion of the anode. U.S. Pat. No. 4,551,400 to Sapru et al. entitled "Hydrogen Storage Materials and Method of Sizing and Preparing the Same For Electrochemical Applications" describes a hydrogen storage material suitable for use as an anode which is a single or multiphase Ni-Ti-V alloy additionally containing Al, Zr or Cr. Furukawa et al. disclose a hydrogen adsorbing anode containing CaNi.sub.5-x Al.sub.x and/or CaNi.sub.5-x Mn.sub.x in Japanese patent application 84/29,194. Kawano et al. teach a multicomponent anode consisting of a hydrogen absorbing alloy powder such as LaNi.sub.5, a powder catalyst for oxygen ionization, a fluororesin powder and an alkali-resistant resin powder in Japanese patent application 84/30,806.
While these materials may have actual charge storage capacities that are superior to conventional electrochemical energy storage materials and do not suffer from concerns inherent with more conventional electrode materials, their costs are higher.
Thus it is seen that the potential exists to optimize materials for energy storage in terms of efficiency and cost. What is needed in this field are economical cell designs having high energy storage densities and the ability to cope with hydrogen and oxygen generation.
It is therefore one object of the present invention to provide an electrode additive for electrochemical storage having the ability to absorb hydrogen and recombine oxygen.
This and other objects of the present invention will become obvious to one skilled in the art from the following description of the invention and the appended claims.